Our invention relates to a motor drive method, and more particularly to a method of, and an apparatus for, driving a reversible alternating-current (a.c.) motor such as a polyphase induction motor for accurate positional control of its rotor in moving any external driven member coupled thereto from one standstill position to another. Our invention represents an improvement of the method described and claimed in Takahashi U.S. Pat. No. 4,763,060, dated Aug. 9, 1988, so that we will incorporate its contents herein insofar as is necessary for a full understanding of our instant invention.
In some industrial applications of electric motors, such as industrial robots and numerically controlled machines, the need often arises for quickly moving a motor-driven member to a desired position and for arresting the motor rotation when the driven member arrives exactly at the desired position. A familiar method of such motor control is such that the difference between the desired and actual positions of the revolving part or rotor of the motor is constantly monitored. The motor speed is increased if the positional difference is large, and decreased if it is small.
A more refined system of motor position control was proposed by us in the paper entitled "High Resolution Servo System of an Induction Motor Using Linear Mode Sliding Control" at the 1988 national convention of the Japanese Institute of Electrical Engineers. We suggested in this paper to predetermine the curve representative of the ideal relationship between the positional difference and the motor speed, in order to assure accurate motor speed control in proportion with the positional difference. The positional difference and the motor speed were both gradually reduced to zero according to this ideal curve.
One of the present applicants, Takahashi, also proposed an a.c. motor drive method in the above cross-referenced U.S. Pat. No. 4,763,060. This known method employed a pulse-width-modulated (PWM) inverter for the speed control of an a.c. motor. Also employed were a set of memories for storing data representative of forward and reverse voltage vectors for creating and controlling a rotary field vector in the motor, and of zero vectors for arresting the rotation of the field vector. The PWM inverter switches connected to the a.c. motor were automatically turned on and off in accordance with the forward and reverse voltage vector data and zero vector data read out in a controlled sequence from the memories.
The method served its intended purpose, however, only to the extent that it made possible the use of a.c. motors in place of more expensive direct-current (d.c.) servomotors or stepper motors. There have been consistent demands in manufacturing and other industries for more advanced motor drive systems.